1. Field of the Invention
This invention is directed to: test apparatus into which a sensor test probe is insertable into a chemical to be tested; the calibration of sensors; to the maintenance of clean or sterile conditions during calibration; and, in one aspect, to optical blood gas sensor calibration devices and methods.
2. Description of Related Art
The prior art discloses a variety of chemical sensors including electrode sensors and electrical biosensors such as electrodes with a layer or layers of biological sensing material and electrodes with a particular sensing material (e.g., material that responds to the pH of a solution; material that responds to the oxygen concentration of a solution; and material that responds to the carbon dioxide concentration of a solution). These sensors are used in a fluid medium, in a gaseous medium, or in a fluid medium with gas diffused in it.
Sensors are calibrated to determine sensor response at two (or more) known analyte concentrations so that the value or level of an unknown concentration can be accurately predicted, measured, and displayed. Generally in the calibration of such sensors it is desirable to expose the sensor to a known medium while isolating it from the remainder of the environment. Often it is desired to maintain the sterility of a sensor during calibration to avoid the need for resterilizing the sensor before use. A variety of problems are encountered when a sensor is to be calibrated with respect to two or more analytes.
The prior art discloses a variety of methods and apparatuses for calibrating optical sensors. Non-clinical sensors are calibrated in multiple containers since sterile conditions and packaging are not a concern. Clinical laboratory sensors are generally calibrated by using a standardized sensor test fluid. For example, calibration of the Orion BOD sensor and Corning Limited 150 ion analyzer requires that an electrode calibrated in a first fluid (buffer) must be rinsed with a second fluid (buffer) or deionized water prior to immersing the electrode in the second buffer. Then the electrode is rinsed with a sample solution or deionized water to obtain a measurement for calibration. Another rinse step is required for each additional sample. Similar multiple rinsings are required for activity calibration. A similar machine that moves the fluid around instead of out of container is required for the machine-controlled calibration of the Ciba-Corning 278 Blood Gas System. Here again, sterility is not a problem.
In-line extracorporeal sensors are calibrated by bubbling a stream of gas through an aqueous solution in contact with the sensors. The sensor is then separated from the test solution (blood) by the addition of a semipermeable membrane to control infection.
Arterial blood gas measurement is one of the single most important laboratory tests of critically ill patients. Measurement of acid-base status, along with oxygen and carbon dioxide levels in arterial blood, is necessary to every medical specialist practicing in an intensive care unit; particularly in administering rational oxygen therapy, managing mechanical ventilation, and evaluating renal disturbances and shock. Inaccuracy in blood gas analysis can result from factors related to the operation and performance of the blood gas analyzer, including calibration techniques for use with optical blood gas sensors. There are a variety of problems with such sensors. For example, U.S. Pat. No. 4,739,645 discloses a complex device for calibrating blood gas sensors in which gas is bubbled through a two chambered vial which provides a recirculating mechanism to prevent fluid from leaving the device. The patent describes a calibration vial for storing and calibrating, under sterile conditions, a gas sensor that comprises an optical fiber whose sensing end carries a sensing element, such as a chromophore or fluorophore. The vial contains a calibrating liquid and is intended to keep the end of the sensor wet and permit fast calibration of the sensor without spillage of the calibrating liquid. The vial has a tubular inner calibration chamber in which the end of the fiber resides and which has a sterile filter-plugged gas inlet in its bottom and an outer concentric calibrating liquid reservoir chamber. The bottoms of the two chambers are interconnected by liquid reflux ports that are positioned so that gas coming into the bottom of the calibration chamber aspirates liquid from the reservoir chamber into the calibration chamber resulting in mixing and circulation of the liquid upwardly through the calibration chamber. The upper ends of the chambers are interconnected by a passageway through which liquid is returned to the reservoir and spent gas exits the calibration chamber. The spent gas is vented to the atmosphere via a vent that opens into the top of the reservoir chamber above the liquid level therein.
There has long been a need for an optical sensor calibrator in which known concentrations of a target substance are produced in the vicinity of a sensor and which eliminates the need for mixing volumes of fluids or changing containers to produce sequential known test solutions. There has long been a need for a calibration device which makes it possible to maintain the clean or sterile condition of a sensor during calibration. There has long been a need for such a sensor which is to be packaged in a static fluid container without the provision for adding an additional volume of fluid. There has long been a need for such a calibrator which does not require that a probe be transferred between multiple containers of premixed liquid solutions.